Fout?!?!

Ik heb deze "onlineteller" maar hij lijkt niet te werken is alles wel goed?
<?
$online_check_sql = "SELECT * FROM online WHERE ip = '" . $_SERVER['REMOTE_ADDR'] . "'";
$online_check_res = mysql_query($online_check_sql);
$online_check = mysql_num_rows($online_check_res);

if($online_check == 0)
{
$online_add_sql = "INSERT INTO online SET ip = '" . $_SERVER['REMOTE_ADDR'] . "', tijd = '" . time() . "'";
$online_add_res = mysql_query($online_add_sql);
}
else
{
$online_change_sql = "UPDATE online SET tijd = '" . time() . "' WHERE ip = '" . $_SERVER['REMOTE_ADDR'] . "'";
$online_change_res = mysql_query($online_change_sql);
$online_offline_sql = "DELETE FROM online WHERE tijd < '" . time() . " - 300'";
$online_offline_res = mysql_query($online_offline_sql);
}

$online_sql = "SELECT * FROM online";
$online_res = mysql_query($online_sql);
$online = mysql_num_rows($online_res);
?>

<li>Online : <?= $online; ?></li>